The present invention relates to a process for the protection of flexible silicone bi-constituent membranes, more particularly flexible self-demolding silicone membranes, particularly in the case of the use of molding by resin transfer.
Molding by resin transfer consists in preparing forms having facing surfaces generating a volume having the profile of the piece to be produced, placing on the walls of this volume a flexible silicone membrane to permit demolding of the finished piece, and injecting resin into the space delimited by said membrane. The walls of the mold can be heated so as better to control the polymerization reaction.
This process is very useful in the automotive industry and in the field of sports and leisure. It permits obtaining better technical performance, reducing the production cycles.
Initially useful only for pieces of small dimensions, this process tends to be more generalized and to find applications including in the production of pieces of large structure as in the aeronautical field.
Epoxide resins are the most widely used. They serve particularly for the production of structural members.
The resins of the phenolic type have particularly interesting advantages in the field of aeronautics, because they are self-extinguishing and the smoke emitted, of small volume, is not harmful. They thus permit satisfying the fire and smoke requirements for material.
Each resin is sold with its catalyst and there is more particularly known one of them which contains para-toluene sulfonic acid.
The flexible bi-constituent silicone self-demolding membranes used in molding by resin transfer are obtained by cross-linking of a silicone rubber in the presence of a catalyst. In the technical field in question, the expression xe2x80x9csilicone rubberxe2x80x9d is used conventionally to designate a polycondensate of polysiloxane of high molecular weight cross-linkable with heat. The expression xe2x80x9cvulcanizationxe2x80x9d is generally used to designate cross-linking.
Such a silicone rubber is a linear polymer constituted by recurring siloxane units carrying substituents of the alkyl or aryl type, said polymer bearing at one of its ends a group comprising a vinyl function which is capable of reacting in a radicular way, and at the other end a non-reactive group under conditions of polymerization radicularly, said blockage group. The substituents of recurrent siloxane units as well as the blockage group are generally selected independently from each other from alkyl radicals having preferably 1 to 5 carbon atoms, fluorinated alkyl radicals having preferably 1 to 5 carbon atoms, phenyl radicals possibly having a substituent selected from linear or branched alkyl radicals having at most 5 carbon atoms, naphthyl radicals. For the blockage group, there is generally preferred a methyl group. For the recurrent substituent units, there is preferred methyl or alkyl radicals having a trifluoromethyl terminal group.
The substituent which comprises a vinyl group and which permits cross-linking the silicone rubber in a radicular way in the presence of a peroxide (for example benzoyl peroxide, dicumyl peroxide or lauroyl peroxide), can be for example a vinyl, an allyl, a butene-3-yl or a pentene-4-yl.
Among the silicone rubbers corresponding to the above formula, can be cited:
VMQ rubbers (polydimethyl/vinylmethylsiloxane) whose chain is constituted by xe2x80x94Si(CH3)2Oxe2x80x94 units and whose end groups are on the one hand vinyl and on the other hand methyl.
PVMQ rubbers (polydimethyl/vinylmethyl/-diphenylsiloxane) whose chain is constituted by xe2x80x94Si(CH3)2Oxe2x80x94 units, xe2x80x94Si(xcfx86)2xe2x80x94Oxe2x80x94 units, in which xcfx86 represents a phenyl group, and whose end groups are on the one hand vinyl and on the other hand methyl
FVMQ rubbers (poly xcex3 trifluoropropylmethyl/-vinylmethylsiloxane) whose chain is constituted by xe2x80x94Si(xe2x80x94CH2CH2CF3) (xe2x80x94CH3)xe2x80x94Oxe2x80x94 units and by xe2x80x94Si (xe2x80x94CH3) (xe2x80x94CHxe2x95x90CH2)xe2x80x94Oxe2x80x94 units, and whose end groups are on the one hand vinyl and on the other hand methyl.
The silicone rubbers identified by the VMQ, PVQM and FVMQ denominations are defined by the ASTM D 1418-83 standard.
A bi-constituent silicone material of the prior art used for producing demoldable membranes is a material obtained after cross-linking a silicone rubber as defined above. It is constituted by chains analogous to those of silicone rubber, two chains being interconnected by at least one xe2x80x94Cxe2x80x94Cxe2x80x94Cxe2x80x94 connection between an Si atom and one of the chains and an Si atom of the other chain.
The flexible silicone self-demolding membrane has numerous qualities which are essential for the good practice of the process. There is no machining, it is easy to demold because it suffices to pull on the membrane to peel it relative to the piece, and the surface condition of the obtained piece is excellent.
By contrast, this membrane has an important drawback: in contact with the resin, it degrades by depolymerization of the polysiloxane under the action of oxidating or radicular species. The result is a degradation of the surface condition, this phenomenon being all the greater upon successive uses.
This premature aging makes the membrane lose all its demolding qualities and leads as a result to a decrease in the quality of the surface condition of the piece. This fact requires the user to proceed to replace frequently the membrane, which replacement is hence prejudicial to the economics of the molding process by resin transfer.
It has been noted that one of the possible explanations of the degradation of the membrane could be the action of the phenyl groups of the resin. The simplified chemical equation of such a reaction can be written in the following manner (R1 and R2 represent substituents of the recurring siloxane units of the siloxane rubber, and R represents the resin carrying the OH group): 
Notice the rupture of the silicone chain which marks the degradation of the flexible membrane.
The acid catalysts permits triggering the degradation by rupture of the polysiloxane chain which depolymerizes and it accelerates the degradation of the membranes. There is thus noted a strong decrease in the demolding capacity of the membrane which becomes, during further use, more and more difficult to separate from the piece, which gives rise simultaneously to a degradation of the surface condition of said piece. The degradation starts in limited regions and propagates rapidly over all the surface of the membrane. Statistics show that these membranes can be used about a dozen times.
One solution consists in decreasing the aggressive power of the catalyst, but the results obtained have been less promising and the possibilities of modification are relatively reduced because it is necessary to preserve its primary qualities of initiating and managing the polymerization reaction.